1. Field of the Invention
The present invention relates to a method for microfabricating diamond, including a step for etching diamond which is a suitable material for electronic devices used at high temperatures, at high frequencies, and in high electric fields.
2. Description of the Related Art
Diamond has excellent heat-resistance and has a wide band gap of 5.5 eV. It is generally an insulator, however, it can function as a semiconductor when doped with an impurity. Also, diamond has excellent electrical characteristics, such as a high dielectric breakdown voltage, a high saturation velocity and a low dielectric constant. Because of these characteristics, diamond is expected to be a suitable material for electronic devices or sensors with high output, which are used at high temperatures, at high frequencies, and in high electric fields. Also, diamond has been applied to the following fields: as an optical sensor or a light emitting device for the short wavelength range of ultraviolet rays or the like, because of the wide band gap; as a material for a heat dissipating substrate because of its high thermal conductivity and low specific heat; as a surface acoustic device by making use of the characteristic that diamond is the hardest substance; as an X-ray window or optical material because of its high transmittance and refractive index; and so on. Also, diamond is used as a material for tools and micromachinery because of its high wear resistance.
A single crystal of diamond is obtained by mining for natural diamond or by synthesizing it at high temperatures and pressures. A single crystal of diamond, however, has areas of approximately 1 cm.sup.2 at the largest, and the price is significantly high. Fields in which diamond is industrially used, therefore, are limited to specific ones such as for polishing powder or as an edge-tool for precision cutting.
Conventionally, various methods of diamond film synthesis have been studied, including, for example, a micro-wave chemical vapor deposition (CVD) method disclosed in Japanese Patent Publication Nos. 59-27754 and 61-3320, a high-frequency plasma CVD method, a hot filament CVD method, a direct-current plasma CVD method, a plasma-jet method, a combustion method, and a thermal CVD method. In accordance with these vapor phase synthesis methods, a large area of diamond film can be obtained with low production costs.
It is a requirement that the surface of a diamond be microfabricated by etching when it is to be used as a core material for practical purposes, as disclosed by K. Kobashi, S. Miyauchi, K. Miyata, K. Nishimura, Journal of Materials Research, Vol. 11, No.11 (1996) p.2744. In particular, in order to apply diamond to electronic devices such as field effect transistors and the like, it is a requirement that diamond is microfabricated in the submicron range (1 .mu.m or less), as disclosed by K. Miyata et al, in IEEE Trans. Electron Devices, Vol. 42, No.11 (1995) p.2010.
The conventional method for fabricating a diamond semiconductor device includes the steps of: forming a resist layer by applying a resist (photosensitive material) onto a diamond film or substrate; projecting a pattern which has been delineated on a photomask beforehand onto the resist layer with a light source such as a mercury lamp which emits ultraviolet rays; transferring the pattern onto the resist layer by developing it; and patterning by etching the diamond film or substrate with the transferred pattern being used as a mask. After the completion of the patterning of the diamond film or substrate, the resist layer is decomposed and removed by oxygen plasma.
In the photolithography step where the pattern is transferred onto the resist layer, the i-line having a wavelength of 365 nm which is the emission line of the mercury lamp as a light source of the photolithography, and ultraviolet rays having a short wavelength such as a KrF excimer laser having a wavelength of 248 nm are used. However, differing from silicon, diamond transmits light having a wavelength of 220 nm or more in accordance with the band gap. As a result, after the i-lines having a wavelength of 365 nm or the ultraviolet rays having a short wavelength such as a KrF excimer laser having a wavelength of 248 nm pass through the mask, a part of the lines or rays passes through the diamond film, is reflected from the rear towards the surface, and exposes the resist layer. This causes the problem of the mask pattern being transferred incorrectly.
Further, since a lower limit of a line pattern width which is transferable by photolithography, that is, a resolution limit, depends on a wavelength of a light source, it is difficult to form a pattern having a line width of 100 nm or less even if the i-line from a mercury lamp and short-wavelength ultraviolet rays of a KrF excimer laser or the like are used.
The method for making a silicon semiconductor device generally employs an electron beam or ion beam having an acceleration energy of 20 to 200 keV instead of ultraviolet rays in order to form a fine pattern having a line width of 100 nm or less. In electron beam lithography or ion beam lithography which employs an electron beam or ion beam, a pattern is directly delineated on a resist layer with an electron beam or ion beam without a photomask. As a resist which is suitable for these lithographic techniques, polymethyl methacrylate (hereinafter referred to as PMMA) has been known. However, PMMA has a disadvantage of weak resistance to the plasma etching.
Conventionally, a spin-on glass (hereinafter referred to as "SOG") material has been used as an insulating coating film. Recently, it was proposed that the SOG film be used as a negative resist because the SOG material is dehydrated and condensed by irradiation with an electron beam or ion beam and changes into a silicon oxide which is insoluble in an organic solvent, as disclosed by A. Imai, H. Fukuda, T. Ueno, Jpn. J. Appl. Phys. Vol. 29 (1990) P.2653; Y. Koh, T. Goto, J. Yanagisawa, K. Gamo, Jpn. J. Appl. Phys. Vol. 31 (1992) p.4479.
FIG. 8 is an example of a chemical structure of the SOG material. In FIG. 8, four hydroxyl groups are bonded to a silicon (Si), and this is called a straight-chain SOG material including a chemical structure in which hydroxyl groups are substituted for methyl groups. When a SOG film is used as a resist, an organic solvent, for example, methanol or butanol, is used as a developer. A SOG pattern formed as described above can be used as an etching mask as it is, and also can be used as an overlying resist in a multi-layered resist, as disclosed in Japanese Patent Publication No.3-287163. Also, in phase-shift lithography which is a technique used to improve the resolution of photolithography, a shifter composed of a silicon oxide layer is formed on a photomask, and it is also possible to employ the patterning method using the SOG material in order to correct defects in the fabrication process.
The dehydration-condensation reaction of the straight-chain SOG material, however, progresses gradually in air and in a vacuum as time passes, even without being irradiated with an electron beam or ion beam. Hence, in the conventional patterning method using the SOG material, irradiation with an electron beam or an ion beam, and the development, must be completed within 7 hours after the SOG application, which is disadvantageous to the fabrication process. For example, in an experiment conducted by the present inventors, in which the straight-chain SOG material is used to confirm the problem described above, after applying the straight-chain SOG material to a sample followed by soft-baking at a temperature of 80.degree. C. for 5 minutes, the sample with the SOG material was left to stand. As a result, the straight-chain SOG material was found to already be insoluble in the solvent in 8 hours and could not be provided for patterning.
As described above, the patterning method using the SOG material has a problem in the fabrication process, that is, since the SOG material becomes insoluble as time passes, a series of steps, including the application of the straight-chain SOG material, irradiation with an electron beam or an ion beam, and the development, must be performed promptly. Also, it has another problem, that is, since the dehydration-condensation reaction of the applied straight-chain SOG material progresses as time passes, the unreacted SOG material, which is soluble in the developer, decreases as time passes during the development for patterning, and therefore, the size of the pattern formed by electron beam or ion beam irradiation changes with time.